The arterial supply of blood to the brain is protected by a series of connections between the major arteries, termed the "circle of Willis" after the English physician who first described it in the 17th century.

People with migraine, particularly migraine with aura, are more likely to be missing components of the circle of Willis. Experts once believed that migraine was caused by dilation of blood vessels in the head, while more recently it has been attributed to abnormal neuronal signals.

In this study, appearing in journal PLOS ONE, researchers suggest that blood vessels play a different role than previously suspected: structural alterations of the blood supply to the brain may increase susceptibility to changes in cerebral blood flow, contributing to the abnormal neuronal activity that starts migraine.

"People with migraine actually have differences in the structure of their blood vessels - this is something you are born with," said the study's lead author, Brett Cucchiara, Associate Professor of Neurology.

"These differences seem to be associated with changes in blood flow in the brain, and it's possible that these changes may trigger migraine, which may explain why some people, for instance, notice that dehydration triggers their headaches," Cucchiara said.

In a study of 170 people from three groups - a control group with no headaches, those who had migraine with aura, and those who had migraine without aura - the team found that an incomplete circle of Willis was more common in people with migraine with aura (73 percent) and migraine without aura (67 percent), compared to a headache-free control group (51 percent).

The team used magnetic resonance angiography to examine blood vessel structure and a noninvasive magnetic resonance imaging method pioneered at the University of Pennsylvania, called Arterial spin labeling (ASL), to measure changes in cerebral blood flow.

"Abnormalities in both the circle of Willis and blood flow were most prominent in the back of the brain, where the visual cortex is located," said the study's senior author, John Detre, Professor of Neurology and Radiology.

"This may help explain why the most common migraine auras consist of visual symptoms such as seeing distortions, spots or wavy lines," Detre said.